Articles of athletic footwear often include two primary elements, an upper and a sole structure. The upper provides a comfortable covering for the foot and securely positions the foot with respect to the sole structure. The sole structure is secured to a lower portion of the upper (for example, through adhesive bonding) and is generally positioned between the foot and the ground. In addition to attenuating ground reaction forces (that is, providing cushioning) during walking, running, and other ambulatory activities, the sole structure may influence foot motions (for example, by resisting pronation), impart stability, and provide traction. Accordingly, the upper and the sole structure operate cooperatively to provide a comfortable structure that is suited for a wide variety of athletic activities.
The upper is often formed from a plurality of material elements (for example, textiles, polymer sheets, foam layers, leather, and/or synthetic leather) that are stitched and/or adhesively bonded together to form a void on the interior of the footwear for receiving a foot. More particularly, the upper forms a structure that extends over instep and toe areas of the foot, along medial and lateral sides of the foot, and around a heel area of the foot. The upper may also incorporate a lacing system to adjust fit of the footwear, as well as permitting entry and removal of the foot from the void within the upper. In addition, the upper may include a tongue that extends under the lacing system to enhance adjustability and comfort of the footwear. Further, the upper may incorporate a heel counter to provide stability, rigidity, and support to the heel and ankle portion of the foot.
The sole structure may include one or more components. For example, the sole structure may include a ground-contacting sole component. The ground-contacting sole component may be fashioned from a durable and wear-resistant material (such as rubber or plastic), and may include ground-engaging members, tread patterns, and/or texturing to provide traction.
In addition, in some embodiments, the sole structure may include a midsole and/or a sockliner. The midsole may be secured to a lower surface of the upper and forms a middle portion of the sole structure. Many midsole configurations are primarily formed from a resilient polymer foam material, such as polyurethane or ethylvinylacetate, that extends throughout the length and width of the footwear. The midsole may also incorporate fluid-filled chambers, plates, moderators, or other elements that further attenuate forces, influence the motions of the foot, or impart stability, for example. The sockliner is a thin, compressible member located within the upper and positioned to extend under a lower surface of the foot to enhance footwear comfort.
Sole structures have been developed that utilize a plurality of support members, which, in some cases, may be generally cylindrical, to provide attenuation of ground reaction forces. Such systems can include support members of various sizes distributed about the midsole to provide cushioning and stability that is tailored to each region of the foot including, for example, the forefoot and/or heel region. However, these systems are not adjustable. While a user may, in some cases, substitute a different insole to provide a different cushioning and/or stability characteristics, the majority of cushioning and/or stability attributes are often provided by the midsole rather than the insole. Therefore, once the article of footwear is manufactured, the performance characteristics of the sole structure are substantially fixed because the characteristics of the midsole are not adjustable. It may be desirable to provide some adjustability for the attributes of the midsole in order to provide a higher level of customizability of the performance characteristics of footwear.